The invention relates generally to the realisation of the radio interface in a cellular radio system between the base stations and the terminals. Particularly the invention relates to the arrangement of the transmitted data and to the determination of the transmission power, so that the available data communication capacity can be maximally utilised. The data communication capacity is formed by time and frequency band.
In cellular systems it is common that the radio communication between the base stations and the terminals comprises cyclically repeating frames which are divided into smaller parts, which can be called slots. A frame represents a certain period, during which a certain frequency band is divided to be used by certain connections with the aid of a multiple access method. Common multiple access methods are TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) and FDMA (Frequency Division Multiple Access). In TDMA the slots in the frames are time slots, whereby one frame can contain for instance eight slots, of which each can be allocated to be used by a certain radio connection. In some systems the size and number of the time slots in a frame can vary from one frame to another. In CDMA the slots represent mutually orthogonal or almost orthogonal spreading codes, which during at least on frame can be assigned to a certain radio connection. In FDMA the slots are narrower parts of the utilised frequency bandwidth. There has also been proposed combinations of FDMA, CDMA and TDMA where the frame is divided into time slots, which further can be divided into smaller parts, based on either time, frequency or code.
During a slot allocated to a radio connection it will experience interference or disturbances, which are caused by other radio connections within the area of the same cell or of an adjacent cell. The amount and quality of the interference depends on the number of simultaneous radio connections and on the multiple access method and reuse factor, which describes how close to a certain cell there is another cell using the same radio frequencies or the same spreading codes. In CDMA the interference of the own cell can form an essential part of the whole interference, but in TDMA and FDMA the larger part of the interference comes from other cells.
The radio connections must achieve a certain correctness so that they can transmit data in a desired manner. This can be achieved at a sufficient C/I ratio (Carrier to Interference Ratio), which represents the ratio of the carrier power and the simultaneously received interference power. For prior art cellular radio systems it is typical that the C/I ratio (or the SIRxe2x80x94Signal to Interference Ratioxe2x80x94or the S/Nxe2x80x94Signal to Noise ratioxe2x80x94or the S/(I+N)xe2x80x94Signal to Interference plus Noise ratioxe2x80x94or another corresponding factor) has been defined with a target level, and for each radio connection the transmit power is controlled to be so high that the target level is barely reached. It is not worth to have a higher transmit power than required to achieve the target level of the C/I ratio, because an unnecessary high transmit power consumes electrical energy in the transmitting device and causes interference in other simultaneous radio connections.
The prior art method is not very efficient regarding the optimisation of the radio interface utilisation. If two adjacent base stations in the TDMA system simultaneously initiate a radio connection when these base stations use the same frequency in their cells, then these connections can be involved in a competition situation: both connections alternately increase their transmit powers in order to raise the C/I ratio to the target level, whereby each power increase in the first cell increases the interference in the other cell, and the actions go on in a circle. A similar competition situation may occur in one base station or in two adjacent base stations of the CDMA system between two simultaneous radio connections using different spreading codes.
The Finnish patent application no. 964308 and the corresponding U.S. patent application with the Ser. No. 08/802,645 present a method for sharing radio resources, where one possibility is to co-ordinate the slot reservation situation between adjacent base stations. The application expresses in general terms that when the slots are reserved it is considered whether the connection requires real time data communication or non-real time data communication, or how high transmit power the connection must use due to the distance between the terminal and the base station.
The object of the present invention is a method and a system, with which the data transmission capacity at the radio interface of a cellular radio system can be maximally utilised.
The objects of the invention are attained by considering a combination of all frame structures used by simultaneous radio connections and by dividing the packets relating to the connections into slots of considered frames and/or by selecting the transmit powers of the packets so that the utility function calculated from the considered connections is maximised.
The method according to the invention is according to the first embodiment of the invention characterised in that in the method
a first utility function is generated having a value which depends on the carrier power and the interference power that can be obtained by calculation for the transmitted packets in their mutual transmitting order at that moment,
the packets to be transmitted are arranged in a mutual transmission order corresponding to the extreme value of said first utility function.
The method according to the invention is according to the a second embodiment of the invention characterised in that in the method
a second utility function is generated, whereby the value of this utility function depends on the carrier power and the interference power which can be obtained by calculation for the packets to be transmitted in their mutual transmitting order at that moment, and
transmit powers are selected for the packets to be transmitted so that the transmit powers together correspond to an extreme value of said second utility function.
The invention relates also to a radio system, which according to the first embodiment of the invention is characterised in that it comprises means
for calculating the value of such a first utility function, the value of which function depends on the carrier power and the interference power obtainable by calculation for the transmitted packets in their current mutual transmission order at that moment,
for giving instructions to the transmitting devices to arrange the packets in such a mutual transmission order which corresponds to the extreme value of said first utility function, and
for giving instructions to the receiving devices to receive the packets in such an order, in which the transmitting devices arrange the packets to be transmitted.
The radio system according to the invention is according to a second embodiment of the invention characterised in that it comprises means
for calculating the value of such a second utility function, the value of which function depends on the carrier power and the interference power obtainable by calculation for the simultaneously transmitted packets, and
for giving instructions to the transmitting devices to select such transmit powers for the packets, which powers correspond to the extreme value of said second utility function.
In order to be able to treat as an entity the effects on different connections caused by the slot allocation and the transmit power selection, a sufficiently large part of the radio interface must be considered at the same time. In this patent application the set of frames examined at the same time is called a radio resource knapsack. In principle the invention does not limit the size of the radio resource knapsack, but in order to have a meaningful application of the invention the radio resource knapsack must contain such frames or corresponding units which are transmitted substantially simultaneously and in which parts (slots) can be allocated to different radio connections, so that it is possible to act centrally on the allocations. The maximum size of the radio resource knapsack depends on the number of simultaneously transmitted frames used in the area where the radio connections contained in the frames can cause mutual interference, and on how effective calculation capacity is available for performing the optimisation according to the invention. The size of the radio resource knapsack is further limited by the requirements on signalling in the base station network, as the signalling requirements are proportional to the size: a centrally controlled allocation of the slots in the frames transmitted via different base stations require signalling between the base stations and the device centrally controlling the allocation of slots.
The size of the radio resource knapsack is not necessarily determined as a fixed size, but it can change dynamically. For instance, the system can monitor which part of the system has the highest load, and then form groups of base stations in the area of the currently highest load, whereby the optimisation of the frame allocation is made by the method according to the invention. Then the simultaneously transmitted frames in the group form one radio resource knapsack. The determination of the radio resource knapsack on the basis of the load or any other variable factor can be called adaptive optimisation of the radio resources.
The invention is suited for application in different radio systems, regardless of whether they utilise circuit switched or packet switched connections. However, because the invention relates to the allocation of slots and the selection of the transmit power for the data amount contained in the slot, this requires that the data transmitted by the connections can be divided into parts with the size of one slot (the slots contained in a frame can be of different sizes). In this patent application the information part transmitted in one slot is for short called a packet, regardless of whether it refers to a circuit switched or a packet switched connection. In the transmitting base station acting as the transmitting device a certain number of packets to be transmitted are stored in a certain transmission buffer. In the same way the packets to be transmitted in the up-link direction from the terminals can be thought as of being in a transmission buffer according to their transmission order. The packets can have some mutual order of importance which is represented by the importance value related to each packet: the higher the value the more important the packet is. Further it is assumed that the cellular radio system in question utilises a retransmission protocol of the ARQ type (Automatic Repeat reQuest), according to which the receiving device can ask retransmission of a received packet which contained errors. For modem ARQ practices it is typical that the receiving device uses all (even the erroneously) received versions when it tries to reconstruct the contents of a transmitted packet, whereby it in fact is not necessary to receive a correct packet even once. All retransmissions increase the probability that the receiving device can correctly reconstruct the contents of the packet.
When one knows the used transmit power for each radio connection and the attenuation of the signal between the transmitter and the receiver related to each radio connection are known, then it is possible to calculate which C/I ratio this will provide in each radio connection concerning each presented slot reservation model or radio resource knapsack packing alternative. For a considered packet i transmitted in a certain slot the value (C/I)i of the C/I ratio representing the packet is obtained for the TDMA system by the equation                                           (                          C              /              I                        )                    i                =                                            P              i                        ⁢                          G              ii                                                          ∑                                                                                                                  j                        =                        1                                                                                                                                                j                        ≠                        i                                                                                            ⁢                                  
                                                                              No                  xe2x80x94                                ⁢                BS                                      ⁢                          xe2x80x83                        ⁢                                          P                j                            ⁢                              G                ij                                                                        (        1        )            
where Pi represents the transmit power used to transmit the considered i:th packet, Gii represents the distance attenuation between the transmitter of the i:th packet and the intended receiver, Pj represents the transmit power used to transmit simultaneously the j:th packet, and Gij represents the distance attenuation between the transmitter of the j:th packet and the intended receiver of the i:th packet. The factor No_BS limiting the summing is the number of those devices, which transmit simultaneously with the transmission of the considered packet. In the CDMA system the corresponding equation is obtained by taking into account the packets transmitted in the same cell at the same time as a certain packet as well as the packets transmitted simultaneously in other cells, as well as the (imperfect) orthogonality of the spreading codes and the processing gain in the receiving device.
According to the first embodiment of the invention the radio resource knapsack is packed, or the packets to be transmitted are placed in the slots contained in the frames, so that the so called utility function calculated over the entire radio resource knapsack is maximised, whereby the utility function depends on C/I ratios or on corresponding values representing the ratio between the signal and the interference which can be obtained by calculation. The relative significance of the C/I ratios of certain packets can be emphasised by considering, in addition to the C/I ratio, also the above mentioned importance value related to a packet. According to another embodiment of the invention the packets which have been selected to be transmitted simultaneously are selected such transmit powers that the so called second utility function is maximised, whereby this second utility function is calculated over the simultaneous packets and depends on the C/I ratios obtainable by calculation, or on corresponding values representing the ratios between the signals and the interference. The relative importance of the C/I ratios of certain packets can also here be emphasised by taking into account the above mentioned importance value relating to a packet, in addition to the C/I ratio. Thus the packing of the radio resource knapsack and the selection of the transmit powers results in a multi-variable optimisation problem, where the variables are the location of the packets in the slots of the frames and the transmit power used to transmit them. The invention does not impose limits on the definition of the utility function, if only it will depend on the C/I ratio of the connections or on another quantity representing the amount and/or quality of the signal and on the other hand of the noise or interference.